1. Field of the Invention
The present invention relates to a protective device for use with a superconducting magnet coil connected to a DC regulated power supply for protecting the superconducting magnet coil when the DC regulated power supply or the superconducting magnet coil is subjected to malfunction or failure.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings illustrates one conventional protective device of the type described above. A DC regulated power supply 1 comprises a transformer 2, a rectifier 3, a DC filter 4, and a transistor 5. An AC breaker 6 serves to cut off the input current to the DC regulated power supply 1 upon a malfunction or failure thereof. A superconducting magnet coil 7, or a load, is connected to the DC regulated power supply 1 through a protective resistance 8 for protecting the superconducting magnet coil 7 upon malfunction or failure and a DC breaker 9 for cutting off the output current from the DC regulated power supply 1 when any malfunction occurs.
The operation of the prior protective device is as follows: When the DC regulated power supply 1 fails, due, for example, to malfunction of the rectifier 3 or the transistor 5, the input current to the DC regulated power supply 1 is cut off by the AC breaker 6. Electric energy stored in the superconducting magnet coil 7 is then consumed by the transistor 5. The DC current flowing through the malfunctioning area is reduced to zero in a short period of time without adversely affecting the superconducting magnet coil 7. Upon any malfunction of the superconducting magnet coil 7, both the AC and DC breakers 6 and 9 are energized to cut off the currents. Electric energy stored in the superconducting magnet coil 7 is then discharged into a closed circuit composed of the superconducting magnet coil 7 and the protective resistance 8, and is consumed by the protective resistance 8. Therefore, the DC current flowing through the superconducting magnet coil 7 falls instantaneously to zero, preventing the superconducting magnet coil 7 from more severe damage.
The prior protective device of the foregoing arrangement suffers from a problem when the DC regulated power supply 1 malfunctions, e.g., when the rectifier 3 or the transistor 5 fails. More specifically, upon such a malfunction, the energy stored in the superconducting magnet coil 7 is consumed by the transistor 5, and the DC current flowing through the faulty area is reduced to zero in a short interval of time, but not instantaneously. Such a time delay before the DC current is completely eliminated causes the malfunctioning area to be severely damaged and adversely affects the superconducting magnet coil.
One solution to the above shortcoming would be to actuate the AC and DC breakers 6, 9 simultaneously at the time of a fault in the DC regulated power supply 1. This would cut off the input current to the DC regulated power supply 1 and sever the superconducting magnet coil 7 from the DC regulated power supply 1, permitting the current flowing through the DC regulated power supply 1 to be zeroed instantaneously. Therefore, the stored energy in the superconducting magnet coil 7 could be consumed by the protective resistance 8, and the current through the superconducting magnet coil 7 could be eliminated momentarily.
Generally, liquid helium is employed for cooling the superconducting magnet coil 7. When the current flowing through the superconducting magnet coil 7 undergoes an abrupt change, the liquid helium is liable to evaporate into a gas phase no longer available for use. Therefore, it is not preferable in the protective device to abruptly vary the current flowing through the superconducting magnet coil 7 unless the superconducting magnet coil 7 itself has a fault or unless otherwise dictated by unavoidable circumstances.
Accordingly, the conventional protective device of FIG. 1 has the above-mentioned difficulty in that the DC breaker 9 cannot be actuated to separate the superconducting magnet coil 7 from the DC regulated power supply 1 when the latter suffers from a fault.
A similar prior art device is described in the Proceeding of the Second International Conference on Magnet Technology, Oxford, 1967, pp. 560-563. This device similarly includes a resistor connected in parallel with the magnet field coil and a switch connected between one side of the resistance and the power supply, such that the resistor absorbs the energy stored in the magnetic field when the switch is opened, entirely similarly to the device described above.